The long term objective of this research is to determine the sequential changes in mammary tissue following irradiation that lead to tumor formation. Specifically this research will quantify and characterize the early alterations in growth potential of mammary cells after radiation exposure and characterize the cell populations that emerge with time during progression to the neoplastic state. Further, these studies will compare and contrast the sequential changes in mammary tissue following the chemical carcinogen 7,12-dimethylbenz[a]anthracene with those changes which occur following irradiation. These studies will also determine whether and how radiation exposure affects the dynamics of the carcinogenic process initiated by a chemical carcinogen. These studies will use the cell dissociation technique and the epithelial focus (EF) assay to examine the induction and dynamics of altered mammary cell populations. This approach allows the examination of the cellular changes which occur with time after in vivo exposure by means of relatively new ex vivo/in vivo and in vivo/in vitro techniques. These studies should help illuminate some of the differences between radiation and chemical carcinogens, if any, as well as provide information on the mechanisms of action and radiation in the carcinogenic process. Further, these studies should provide information on the processes involved in mammary carcinogenesis which may be applicable to understanding the development of mammary cancer in humans.